Phonon Thermal Hall Effect in Strontium Titanate

TLDR

Phonons can produce an off‑diagonal thermal conductivity in a magnetic field, yet recent studies have largely neglected their contribution to thermal Hall conductivity. The authors investigate the thermal Hall conductivity κ_xy in the quantum paraelectric SrTiO₃, a non‑magnetic insulator, to assess its magnitude relative to other insulators. They find that κ_xy in SrTiO₃ peaks at the same temperature as the longitudinal κ, decreases more rapidly on either side, exceeds all previously reported values for insulators, shows similar temperature dependence in La₂CuO₄ and α‑RuCl₃, but is only weak in KTaO₃, indicating that antiferrodistortive domains are essential for generating large κ_xy.

Abstract

It has been known for more than a decade that phonons can produce an off-diagonal thermal conductivity in presence of magnetic field. Recent studies of thermal Hall conductivity, $\kappa_{xy}$, in a variety of contexts, however, have assumed a negligibly small phonon contribution. We present a study of $\kappa_{xy}$ in quantum paraelectric SrTiO$_3$, which is a non-magnetic insulator and find that its peak value exceeds what has been reported in any other insulator, including those in which the signal has been qualified as 'giant'. Remarkably, $\kappa_{xy}(T)$ and $\kappa(T)$ peak at the same temperature and the former decreases faster than the latter at both sides of the peak. Interestingly, in the case of La$_2$CuO$_4$ and $\alpha$-RuCl$_3$, $\kappa_{xy}(T)$ and $\kappa(T)$ peak also at the same temperature. We also studied KTaO$_3$ and found a small signal, indicating that a sizable $\kappa_{xy}(T)$ is not a generic feature of quantum paraelectrics. Combined to other observations, this points to a crucial role played by antiferrodistortive domains in generating $\kappa_{xy}$ of this solid.